Wide angle photographic objective



.uRo MUKAI wrm: ANGLE PHOTOGRAPHIC OBJECTIVE Find Feb. 7, 195s FIG. Z

Jag. 31, 1961 INVENTOR. BY d// Nwm/ Amm/ff United States Patent O-P WIDE ANGLE PHOTOGRAPHIC OBJECTIVE Jiro Mukai, Kohoknku, Yokohama City, Japan, asignar to Canon Camera Company, Inc., Tokyo, Japan, a corporation of Japan .Filed Feb. 7, 195s, ser. No. 713,899

1 claim. (ci. sii-s1) This invention relates to a photographic objective. More particularly it relates to an objective having a long back focus.

An object of this invention is to provide a large aperture, wide angle photographic objective which has a relatively long back focus as compared with the effective focal length and whose chromatic aberration, spherical aberration, coma, astigmatism, image curvature and distortion.are well corrected.

A clear concept of the scope and purpose of this invention may be obtained from the following description, in connection with the appended drawing, in which:

Fig. l is a sectional view showing an embodiment of the wide angle photographic objective according to the present invention; and

Figs. 2 to 4 are diagrams showing the spherical aberration and deviation from the sine condition, the astigmaxism and the distortion of said objective, respectively.

The objective of my invention consists of a front negative meniscus lens land a rear group widely spaced from such front lens. The rear lens group consists of at least six lenses and includes an iris diaphragm therein. Each of the lenses disposed in front of the iris diaphragm is a single meniscus lens air spaced from the others. The lens just behind the diaphragm consists of a negative element cemented to a positive element with the concave surface of the cemented lens toward the diaphragm, while the rearmost elem'ent is a positive element. In addition, the objective satisfies the relations:

where R Mm represents the radius of curvature of the lens surface, S subscript the distance along the optical axis of the air space between successive lenses, f the focal length of the objective, and pc the refractive power of the cemented lens of the rear lens group, each series of subscripts increasing from the front to the rear of the objective.

Referring to the illustrative embodiment of Fig. 1, L1 is a negative meniscus front lens, and L2 to Ls are the lenses of the rear lens group arranged with a large air space S1 from the front lens L1. In the rear lens group, the three lenses I4, L and L4 are positive meniscus lenses with their respective convex surfaces di. rected toward the object and withtheir radii of curvature progressively increasing. A relatively strong negative meniscus lens L5 is arranged to the rear of the three lenses L2, L3 and L, with its concave surface toward the image, a diaphragm being positioned just behind lens L5. A cemented meniscus component consisting of a negative lens L, and a positive lens L7 is disposed behind the diaphragm with its concave surface directed toward the diaphragm and a double convex lens Lis provided as the rearmost element of the objective. In the rear 2,969,713 Patented Jan. 31, 1961 lens group, the number of lenses needs not alway's be u in the illustrative embodiment, but there should be at least six or more component lenses in the entire rear lens group and the lenses behind the diaphragm should meet the above ,mentioned requirements as regards the radii of curvature, focal lengths and air spacings.

A concrete embodiment of the objective according to this invention is as follows:

RWM is the radius of curvature of the lens, dwmt the thickness of the lens at the optical axis, Smm the air spacing at the optical axis, Nsubscnpt the index of refraction of the glass of the lens, and VWM, the Abbe number of the glass of the lens, the subscripts increasing in the direction from the front of the objective.

In Fig. 2 the full line graphs show the spherical aberration for the d-line and g-line, respectively, of the instant objective, the lighter dashed-line graph the deviation of the d-line from the sine condition and the heavier dashed-line graph of the g-line from the sine condition. In Fig. 3 the full line graph is the graph of the sagital field curvature of the astigmatism of the objective, and the dashed-line graph that of the meridional eld curvature, while the graph of Fig. 4 is the distortion for each angle of incidence thereof.

It has been customary to arrange a negative meniscus lens as the rst lens in a lens system to obtain an objective with a comparatively long back focus for its focal length. However, the greatest aperture ratio heretofore attainable with this type was only around #2.8. The major cause thereof is that, if the aperture ratio becomes larger than that, not only` will spherical aberration but also comaincrease markedly and cause great practical dicul-ties. On the other hand, according to the present invention, the .rear lens group is formed of six or more lenses, glasses of relatively high refractive indices are used for the positive lenses to make the zonal spherical aberration as small as possible; specically, the lens just behind the diaphragm is a cemented meniscus lens composed of a negative and a positive lens ce.

mented together, with the concave surface directed toward the diaphragm, the absolute value of the radius of curvature of such concave surface being from 0.25 f to 0.9 f, the refractive power of said meniscus lens is from .-S/f to S/f, the ratio of the radii of curvature of both air bounded surfaces is between 0.5 to 1.5, the air space including the diaphragm is between 0.1 to 0.3 f. Coma is thereby effectively corrected. Thus, the present invention provides a wide angle photographic objective having a reiatvc aperture of fz2, which has been hitherto held most dicult to make.

What I claim 1s: A wide angle photographic lens satisfying the following relations:

[f-l 17:2 2er-60 Back focus-0.97)]

Lens Radius o! Thiebness Refractive Abbe Lens Curvature or Air Index Nambe:l

Spacing Iii-2.378 L1.------------ eil-0.044 Tf1-1.6127 Vr- S|1.021 11a-1.634 In. da-LM Nal-Lm Vs-G Ba-(LOM R4H-0.885 14 nlm INTO-1.6127 V3I-58.6

sl-Dm A 1in-'0.556 L4 i4-0.1m NF4-1.6425 V58.

Salam 1 IIe-1.211 Ll dillo-) Nl-Lm VII-35,6

Bul-0.357 l 8F03 A A Rin- 0.625 Le.---..-....-- nig-01H0 bi1-1.6364 Trl-85.4

,Ru-1.21K) 1n--....---...- d1-0.124 'N1-1 V1-55.51

Ris-0.729

S|0.5 11u-2.489 14 d|-0.066 bil-1.6968 7l-55.61

Bil- 2.045 1 objective.

References Cited in the le of this patent UNITED STATES PATENTS Albrecht Sept. 30, 1952 Angenieux Aug. 18, 1953 Tronnier Dec. 15, 1953 Tronnier May 22, 1956 Lange Feb. 5, 1957 Klemt June 18, 1957 Klemt Feb. 25, 1958 

